Optical proximity sensors, such as the AVAGO TECHNOLOGIES™ HSDL-9100 surface-mount proximity sensor, the AVAGO TECHNOLOGIES™ APDS-9101 integrated reflective sensor, the AVAGO TECHNOLOGIES™ APDS-9120 integrated optical proximity sensor, and the AVAGO TECHNOLOGIES™ APDS-9800 integrated ambient light and proximity sensor, are known in the art. Such sensors typically comprise an integrated high efficiency infrared emitter or light source and a corresponding photodiode or light detector, and are employed in a to large number of hand-held electronic devices such as mobile phones, Personal Data Assistants (“PDAs”), laptop and portable computers, portable and handheld devices, amusement and vending machines, industrial or sanitary automation machinery and equipment, contactless switches, and the like.
Referring to FIG. 1, there is shown a prior art optical proximity sensor 10 comprising infrared light emitter 16, light emitter driving circuit 31, light detector or photodiode 12, light detector sensing circuit 34, metal housing or shield 18 with apertures 52 and 54, and object to be sensed 20. Light rays 15 emitted by emitter 16 and reflected as light rays 19 from object 20 (which is in relatively close proximity to optical proximity sensor 10 and within the detection range thereof) are detected by photodiode 12 and thereby provide an indication that object 20 is close or near to sensor 10.
Light rays 15 typically cause light detector 12 to generate small AC current signals that are dwarfed by the ambient light DC current signals generated by ambient light falling on light detector 12 at the same time as rays 15 are being sensed. In some prior art devices, these small AC current signals are separated from the large ambient light DC current signals in a two-step procedure. In a first measurement cycle, LED 16 is turned off, and the DC current signal arising from ambient light falling on light detector 12 is integrated and stored in memory as a first measurement. In a second measurement cycle, LED 16 is turned on, and the AC current signal together with the DC current signal is integrated and stored in memory as a second measurement. To yield only the integrated AC current signal, the first measurement needs to be subtracted from the second measurement.
Such an approach can result in considerable complexity being required to implement the measurement method according to which the two measurement cycles are conducted. In addition, a subtraction operation is required. These requirements decrease the speed of signal acquisition and lower the power efficiency of sensor 10. Moreover, because the DC current signals presented to a subsequent stage analog-to-digital converter (ADC) stage from an amplifier stage can be 20,000 times greater in amplitude than those of the AC current signals, which reduces the dynamic range over which the desired AC current signals can be measured by the ADC stage. In such a situation, only the last few conversion bits or least-significant-bits (LSBs) are dedicated to detecting the narrow range of amplitudes over which the AC current signal is to be found, while most of the base conversion bits or most-significant-bits (MSBs) are dominated by the relatively fixed DC current signals.
This results in poor utilization of the full scale input dynamic range available in the ADC, as only a limited portion of the dynamic range is dedicated to quantifying the AC current signals. The subtraction operation also requires attempting to resolve the differences of two nearly equal large numbers corresponding to the DC current signals while attempting to obtain small numbers representative of the AC current signals. Such steps require the use of a very high resolution ADC, which in turn results in increased complexity and higher implementation costs.
What is needed is a differential integrator circuit that may be employed in applications such as optical proximity sensors that has increased dynamic range and sensitivity, and reduced complexity.